There are many computer-controlled actuators available for robotic and related applications. Examples of such actuators include PVDF actuators, piezoelectric actuators, and electroheological fluid actuators. Other common actuators are linear motors, electro-magnetic actuators, hydraulic actuators, pneumatic actuators, and explosive actuators. These prior art actuators suffer from various disadvantages such as size, complexity, high weight and weight/displacement, large power requirements, and high material costs.
It is known that certain co-polymers can be chemically contracted and expanded in electrolytic solutions by varying tile degree of ionization of the solution, or the pH. As originally reported by W. Kuhn. B. Horgitay, A. Katchalsky, and H. Eisenberg, "Reversible Dilation and Contraction by Changing the State of Ionization of High Polymer Acid Networks," Nature, Volume 165. Number 4196, pages 514-516 (1950) a three dimensional network consisting of polyacrylic acid can be obtained by heating a foil of polyacrylic acid containing a polyvalent alcohol such as glycerol or polyvinyl alcohol. The resulting three dimensional networks are insoluble in water but swell enormously in water on the addition of alkali, and contract enormously on the addition of acids. Linear reversible dilations and contractions on the order of more than 400 percent have been observed. Furthermore, the ultimate structural deformation of these gels is homogeneous in the sense that, for example, for a long cylindrical gel, the relative changes of the length and diameter are the same. Similar properties are exhibited by polymethacrylic acid cross-linked by divinyl benzene co-polymerized in methanol.
These effects can also be obtained electrically if a conductor is included in the polymer. Applying a voltage across the polymer gel causes a pH gradient to evolve between the electrodes. A reversible expansion and contraction of the gel is obtained with the application of an electric field. Direct motion control of the polymer is therefore feasible. The behavior of polymeric gels in an electric field is discussed by T. Tanaka. I. Nishio. S. Sun, and S. Ueno-Nishio. "Collapse of Gels in an Electric Field," Science, Volume 218, pages 457-469 (1982). In principle, the devices of the prior art need have only one moving part, the actuating gel itself. There is not the attendant weight and complexity of electric motors or hydraulic pumps and actuators. All that is required is an electric field on the order of a few volts per centimeter. The major disadvantages of such devices are that, in general, the response times of the gels are much longer than conventional actuator components, and there is the inconvenience that the gel must be contained within a solvent bath.
Practical polymeric gel actuators are described by Adolf Segalman, Shahinpoor, and Witkowski in commonly assigned U.S. patent application No. 07/902,322. In these actuators, flexible containers are used to contain the electronic solution while still allowing movement. Electrodes are mounted with the container so that current may be introduced through the electrolytic solution. The ions formed by the current result in changes in the dimensions of the polymeric gel. The dimensional changes of the polymeric gel are converted into mechanical actuation outside the flexible container. Unfortunately, actuator performance is dictated by the parameters of the polymeric gel used.
The forces and rates of expansion and contraction in the actuators described by Adolf et al are dependent on the particular polymeric gel used in the actuator. Although there are a number of polymeric gels suitable for actuators, they represent a limited range of potential actuator parameters. There are gels that expand and contract approximately equally, and gels that exert more contraction force than expansion force. These differing gels are useful in actuators as long as their relative expansion and contraction forces and rates match those desired for the actuator. While the achieved actuator performance characteristics can be modified by varying the size and shape of the gel, the actuator's relative rates and forces remain those of the gel itself. There exists a need for actuators to serve in applications requiring different relative forces or rates than the gels display.